Attention has recently been drawn to display devices mounted with GUIs (Graphical User Interfaces). The GUIs are achieved by incorporating input devices called touch panels into display devices such as liquid crystal display devices. The GUIs serve as alternatives to common mechanical buttons. The GUIs enable information input by causing the display devices to display various virtual keys.
There are several types of touch panel such as an optical type and a resistive type. Especially in portable terminals, capacitive touch panels are commonly used, as they have comparatively simple structures and consume less electric power.
However, a capacitive touch panel had such a problem that there is a possibility that noise attributed to an inverter fluorescent light, AM (amplitude modulation) waves, an AC (alternating current) power source, or the like (such noise being hereinafter referred to as “external noise”) may be transmitted to the touch panel to cause a malfunction.
This malfunction is attributed to the fact that a capacitive touch panel cannot distinguish between a signal for indicating the presence or absence of a touch that is generated when a user touches the touch panel with a finger or the like or moves a finger or the like toward the touch panel (such a signal being hereinafter referred to as “touch signal”) and external noise. Usually, a capacitive touch panel has its transmitting circuit output a signal for driving the touch panel (such a signal being hereinafter referred to as “driving signal”), has its receiving circuit perform detection at a timing when the driving signal rises or falls, and subjects the received signal to A/D (alternating current/direct current) conversion. If a cycle of this detection and a cycle of external noise interfere with each other, the touch panel may become unable to distinguish between the touch signal that is supposed to be detected and the external noise.
In order to solve this problem, Patent Literature 1 proposes a method for detecting a touch signal synchronized with a driving signal for driving a capacitive touch panel, in which method a touch signal is detected under a condition that is free of the influence of external noise, the condition being selected using a plurality of signals of different frequencies. In this method, the receiving circuit detects a touch signal by using a plurality of driving signals of different frequencies to select, from among these driving signals, a signal of a frequency that is free of the influence of external noise.
Next, in Patent Literature 2, correction data corresponding to a detection timing indicated by a detection timing signal is obtained from a correction data table in synchronization with a display timing signal, and a correction circuit corrects detected data by using the correction data thus obtained.
Further, in Patent Literature 3, a drive control circuit controls the driving signal to be applied so that a detected signal is a polarity alternate signal containing a peak-to-peak asymmetric signal component attributed to the presence of an external object nearby.
Furthermore, in Patent Literature 4, the moving average of contact information is calculated for even numbers of successive frames, whereby calculated contact information in each of the frames can be obtained.